A vaccine that protects women against cervical cancer has proven a tougher sell than health workers had hoped, with vaccination rates stalling from 2011 to 2012.

Since 2006, the Centers for Disease Control and Prevention (CDC) has recommended that all girls 11 and up get three doses of the human papillomavirus vaccine, which protects against 70% of cervical cancers that can appear 20 to 40 years later. The vaccine also protects against 90% of genital warts.

Vaccination rates increased substantially in the first five years the vaccine was available. By 2011, 53% of girls had gotten at least one dose of the HPV vaccine. However, in 2012 the increase stopped, staying at just 53.8%, according to a paper in this week's edition of the Morbidity and Mortality Weekly Report published by the CDC in Atlanta.

HPV vaccination "coverage for girls getting this anti-cancer vaccine has not increased at all from one year to the next. Zero," CDC Director Tom Frieden said in a telephone briefing Thursday. "We're dropping the ball. We're missing opportunities to give the HPV vaccine. That needs to change to protect girls from cervical cancer."

The fact it hasn't increased "is a huge disappointment, but I'm confident that we will turn it around," Frieden said.

The vaccine is highly effective. A study released in June found that the vaccine had decreased the incidence of the cancer-causing virus among teenage girls by 56%, despite being available only since 2006.

The HPV vaccine comes in a series of three shots over six months or longer. CDC recommends that both girls and boys receive all three doses before they become sexually active. Because the recommendation for boys was only added in 2011, data were only collected for girls.

The reasons behind the standstill are varied. A 2012 survey of families with teen girls who said they didn't plan on having their daughters vaccinated found that 19% said their daughter didn't need the vaccine, 14% hadn't had the vaccine recommended to them by their doctor, 13% had safety concerns about the vaccine, 12% didn't have knowledge of the vaccine or the disease and 10% said their daughter wasn't sexually active.

"We asked parents why they haven't gotten their daughters vaccinated, and one of the top reasons is that their doctor didn't recommend it," Frieden said.

"It's up to doctors to have open, honest and frank discussions with parents about the importance of this vaccine and to ensure their patients get vaccinated," said Thomas McInerny, president of the American Academy of Pediatrics, also during the briefing.

"This vaccine is preventive," said Shannon Stokley, one of the paper's authors. She is with the National Center for Immunization and Respiratory Diseases. "If you do have an infection later in life, you can't get the vaccine to make it go away. We're so lucky that we have a vaccine to prevent these cancers — there aren't many diseases where we can do that."

If every girl 11 and up who saw a health care worker since 2007 had been encouraged to get the HPV vaccine, coverage could have reached 92%, the paper stated.

Some parents have expressed concerns that this vaccine may be viewed as "a license or permission to have sex, but multiple studies have found that preteens and teens who receive this vaccine do not have sex any sooner than their peers who have not received the vaccine," Frieden said. "HPV vaccine does not open the door to sex. HPV vaccine closes the door to cancer."

As for young women not being sexually active, Frieden said the vaccination is given to younger girls to protect them when they become adults, just as children are vaccinated for measles well before a child may get exposed.

Fears about the safety of the vaccine may have been stoked by anti-vaccination websites claiming that thousands of girls have had serious reactions to the HPV vaccine and more than 100 have died.

Those numbers do not appear to be based on actual data. In the United States, vaccine safety monitoring is conducted by federal agencies and vaccine manufacturers. Out of the more than 56 million doses of the HPV vaccine distributed since 2006 there had been only 21,194 adverse events reported, mostly pain at the site of the shot, redness and swelling. Reports classified as "serious" adverse events included headache, nausea, fainting, dizziness, vomiting and generalized weakness. There were 1,674 reported instances of these. There have been no deaths linked to the vaccine, said Cindy Weinbaum, a medical epidemiologist with CDC's immunization safety office,

The HPV vaccine "works. It works even better than we had hoped," Frieden said. "HPV vaccine is safe. And it's effective at reducing cancer."

Certain diseases appear during adolescence, around the same time as the HPV and other vaccines are given. Some parents may worry that the two are linked. However, Weinbaum said they have been unable to find any links. "We've been doing a systematic review of medical records in managed care systems for any adverse events that occur after vaccination with the HPV vaccine and we haven't seen anything other than fainting."

But that's the case with all vaccines given to teens, said Weinbaum. "Adolescents receiving vaccines faint more than infants. That also happens with other vaccines given to teens, such as TDAP and the meningococcal vaccine," she said.

About 79 million Americans, most in their late teens and early 20s, are infected with the human papillomavirus. Each year, about 17,400 women in the United States get cancer caused by the virus, with cervical cancer being the most common type.

Men can also get cancer from the human papillomavirus. Each year about 8,800 men get these cancers, mostly in the throat. The CDC began recommending in 2011 that boys over 11 also get the vaccine.